1. Field of the Disclosure
This application generally relates to using a fractional-N synthesizer with tuning of a voltage controlled oscillator (VCO), and other matters.
2. Background of the Disclosure
Some systems using electronic components find it necessary or convenient to generate signals having a programmable known frequency. For example, phase-locked loops (PLL's) are known for use in adjusting the frequency of a first signal to match a second signal frequency or multiples of second signal frequency with a programmable known frequency.
One problem that has arisen in the art is that VCO's often, if not substantially always, need to have their output signal's amplitude adjusted, to avoid generating a signal that is the correct frequency, but is too low in amplitude (that is, not energetic enough) or too high in amplitude (that is, is too energetic). Known systems sometimes address this problem by adjusting the amplitude of the output signal up or down to fall within a range of output signal energy known to be suitable. Fine control of the output signal's frequency could then be adjusted with PLL's or similar electronic circuitry. Mechanisms which continuously control and adjust the VCO amplitude often add noise to the VCO. For this reason, it would be better to set the amplitude of the VCO discretely in time. However, amplitude of oscillation in the VCO will also depend on the VCO frequency. So, if after setting the VCO amplitude, its frequency is change, it will change the amplitude of oscillation too.
While this method of adjusting the amplitude of the output signal up or down to fall within a known range of output signal energy could address some of the problems associated with coarse frequency control, it could also be subject to several drawbacks. A first drawback is that if the output signal's energy is too high, the transistors and capacitors in the VCO could work in a region that harms them. An aspect of this problem is that of possibly damaging the electronic circuitry, either in the VCO or in the downstream circuitry, such as possibly by overpowering the circuit elements.
A second drawback is that if the output signal's energy is too low, the output signal from the VCO could be insufficiently strong to operate the downstream circuitry, or it may be noisy. An aspect of the problem of adjusting the amplification at the output of the VCO is that lowering the amplified power output to less than a known threshold, alone, might fail to lower the total of a DC bias and the amplified output to less than that known threshold. Thus, with the limitation V_DC+V_AMP<V_theta, where V_DC is a DC voltage, V_AMP is oscillation amplitude voltage of the VCO, and V_theta is a threshold voltage. In most cases, the total voltage of V_DC+V_AMP defines if the VCO devices are operating in the safe region or not. Therefore, Detecting only V_AMP to adjusting this voltage may result in an suboptimal amplitude setting for the VCO.
3. Some Drawbacks of the Known Art
Each of these issues, as well as other possible considerations, might cause difficulty in aspects of tuning oscillators, including oscillators designed to output a programmed known frequency, including voltage-controlled oscillators (VCO's).